Many small brush type electric motors, such as fractional horsepower universal type motors and miniature dc motors have brush cages for guiding brushes into contact with a commutator. It is common for the brush cages to be essentially brass tubes in which the brushes slide under the influence of a brush spring. The brush tubes are fixed to the motor structure to ensure accurate location of the position of contact between the brush and the commutator. Often the motor structure includes a bearing bracket of plastic or similar insulating material and the brass tubes are pressed into holes in the bearing bracket. Alternatively, when the bearing bracket is electrically conductive, an insulated bushing is used to fix the brass tube to the bearing bracket.
The brush tube requires means for fixing it within the hole in the insulator. This is usually in the form of a step which prevents the tube from being pressed in further and a raised finger which is resiliently deformed as the tube is being pressed in but snaps into a recess as the tube is fully inserted to prevent withdrawal.
The brass tube is a precision assembly. If the tube's internal dimensions are too small, the brush will not slide and if it is too big, the brush will wobble within the tube causing sparking on the commutator leading to rapid wear of both brush and commutator. Also, the hole in the insulator must be tightly controlled so that the tube will be held snugly but not so tightly to deform the tube and catch the brush. Another disadvantage is that the snap finger is not very resilient and often does not rebound enough to adequately hold the tube in place. To overcome this problem, the recess for the finger is made bigger to ensure clearance between the finger and the edge of the recess but this allows axial movement of the brush tube which also leads to sparking under vibration.
While some motors do not use a brass brush cage but allow the brush to slide directly in the through hole of the bearing bracket, this is not acceptable for high speed and high power motors where positioning and heat dissipation are critical issues.